The present invention relates to a battery such as a primary battery or secondary battery and in particular relates to improvements in a metal outer can for a battery having a cylindrical or prismatic shape.
In recent years, as portable equipment has become increasingly common, the demand for miniature primary batteries and secondary batteries has grown. As primary batteries, manganese dry batteries or alkali manganese dry batteries or lithium batteries are primarily used, depending on the respective application. Also, as secondary batteries, considerable use has previously been made of nickel-cadmium accumulators constituting alkali accumulators in which an aqueous solution of alkali is employed as electrolyte and nickel-hydrogen rechargeable batteries, in which a hydrogen-absorption alloy is employed as negative electrode. Recently however, lithium ion secondary batteries using an organic electrolyte, which have the characteristic advantage of light weight, have suddenly appeared on the market.
In addition to the cylindrical and coin-shaped types, which were the typical shapes conventionally used for batteries, principally miniature secondary batteries for portable equipment, in recent years use of prism shaped batteries has started to increase and, in addition, paper-form thin batteries are currently starting to appear. An important recent trend regarding the performance demanded for these batteries is increased energy density of the battery. Speaking in general terms, there are two ways of indicating battery energy density. One of these is volume energy density (Wh/l); this is employed as an index of miniaturization of the battery. Another is weight energy density (Wh/kg); this is employed as an index of weight reduction of the battery.
Such batteries having high volume energy density (Wh/l) or weight energy density (Wh/kg), which are indices of miniaturization and weight reduction, are prized by market demand and there is fierce competition regarding battery energy density in all battery types.
What primarily determines the level of battery energy density are the battery active materials of the positive electrode and negative electrode that constitute the elements for electromotive-force; apart from these the electrolyte and separator are also important. Improvements of these elements aimed at increasing battery energy density are currently being made in an extremely active fashion.
Another aspect, which used to be overlooked, is the battery casing, i.e., the metal outer can of the battery, which accommodates these elements for electromotive-force. However, in recent years this has been re-evaluated as an important problem and improvements are actively being sought.
If the outer can of the battery can be made thinner, it is possible to accommodate more battery active material in such a member that is thinner but having the same conventional shape, thereby enabling the volume energy density of the battery as a whole to be raised. Also, if the outer can of the battery can be made of lightweight material having a lower specific gravity, a reduction in weight for the same conventional shape can be achieved, thereby reducing the weight of the battery as a whole, and making it possible to raise the weight energy density of the battery as a whole.
The main conventional method of manufacturing battery cans (metal outer cans) was to manufacture the battery can having the prescribed shape by repeating a plurality of deep-drawing steps performed by a press machine (herein this is termed the "transfer drawing method"). However, the DI (drawing and ironing) method has attracted attention as a technique that should be particularly mentioned in connection with the manufacture of battery outer cans, whereby a further reduction in thickness of the battery can be achieved and the volume energy density raised, and also as a technique whereby productivity may be further improved. Specifically, conventionally drawing was usually employed for battery can manufacture, but advantages are obtained by the DI (drawing and ironing) method, in which both drawing and ironing are employed (see Published Japanese Patent No. H.7-99686).
This DI method is a method in which, as is known by Published Japanese Patent No. H.7-99686 etc., a cup-shaped intermediate product is manufactured by a deep drawing step by a press machine, and a battery can having a cylindrical shape having a bottom of prescribed shape is manufactured continuously in a single step from this cup-shaped intermediate product by an ironing step using an ironing machine. In comparison with the "transfer drawing method" it has the advantages of improving productivity due to a decrease in the number of steps, a reduction in weight and an increase in capacity due to decreased thickness of the can side circumferential wall, and reduction in stress corrosion. Its use is therefore increasing. Conventionally, in this method of manufacture, nickel-plated steel sheet having a comparatively high hardness was employed for the battery can blanks, in order to ensure sufficient pressure-resistant strength of the battery can and sufficient strength of the sealing aperture. This DI method makes it possible to reduce the thickness of the battery outer can and makes it possible to achieve an improvement in the volume energy density of the battery of about 2.about.5%.
Although there is some variability depending on battery size, materials used, and method of manufacturing the battery can etc., the weight ratio occupied by the metal outer can in the weight of the battery as a whole in batteries that have been actually used previously is about 10 to 20 wt. % in the case of a cylindrical nickel-hydrogen rechargeable battery or lithium ion secondary battery and, in the case of a prismatic nickel-hydrogen rechargeable battery or lithium ion secondary battery, at about 30 to 40 wt. %, this has a value about twice that for the cylindrical type. In particular, this value was high for the prismatic type in view of the difficulty of ensuring pressure withstanding strength of the battery can.
Such movement to miniaturization and weight reduction of battery casings, i.e., the metal outer can of the battery, is effective in raising battery energy density, as described above, but in actual battery use involving charging, discharging or leaving to stand etc., as well as energy density reliability of quality and safety are important and cannot be ignored. In the case of primary batteries which are used for discharge only, guaranteed capacity and prevention of liquid leakage even when stored for a long period, as well as reliability of quality such as a stable discharge characteristic are indispensable. In the case of secondary batteries that undergo repeated charging and discharging, in addition to the characteristics required for primary batteries, properties such as cycle life and safety are even more important.
Conventionally, in regard to these battery metal outer cans, it was extremely difficult to satisfy requirements for both higher energy density and product quality and safety. Specifically, when it was attempted to increase energy density by some method involving the metal outer can of the battery, it was found that batteries deformed or cracks were produced under abnormal conditions, resulting in frequent occurrence of problems such as leakage of electrolyte. On the other hand, if the metal outer can was made strong, an improvement in energy density was often sacrificed; an effective method of improving the trade-off relationship of these two could not be found.
In the process of manufacturing battery cans indicated above, a method using the DI process involving drawing and ironing is excellent in that it reasonably satisfies demands for increasing the energy density of batteries by making the batteries with a small thickness and light weight and also both quality reliability and safety of the batteries. However, in this respect, improvements such as further increase in performance and quality reliability and stability were sought.
There is a strong market demand for increased convenience of such primary batteries and secondary batteries, i.e., for battery miniaturization and weight reduction. On the other hand, quality reliability and safety of such batteries are indispensable. Conventionally, both quality reliability and safety and the improvement in battery energy density that makes possible miniaturization and weight reduction of batteries were insufficiently satisfied.
Also, if, in the process of manufacturing a metal outer can, extreme miniaturization and weight reduction were attempted, even if a method based on the DI process used for cylindrical batteries produced by drawing and ironing was employed, defects such as corrosion or cracks occurred in the metal outer can, which was not therefore fully satisfactory. Furthermore, in the case of prismatic batteries, which were conventionally manufactured by can drawing, there were problems regarding miniaturization and weight reduction of the metal outer can of the battery.
An object of the present invention is to mitigate the problems described above and, aiming at miniaturization and weight reduction of the metal outer can which has a cylindrical or prismatic or similar shape used in a primary or secondary battery, to provide a battery of improved battery energy density and satisfying battery quality reliability and safety.